Until relatively recently the so-called three-layer technique had to be employed for photolithographic generation of highly resolved structures (sub micrometer range) on substrates with predetermined topography. This technique is, however, very elaborate and cost-intensive. Moreover, it is also not reproducible to any satisfactory degree, in particular due to the irregular formation of the middle protective layer.
In the meantime, high-resolution silicon-containing positive resists have been developed, which are resistant to reactive ion etching in an oxygen plasma, i.e. have the requisite O.sub.2 /RIE resistance (RIE=Reactive Ion Etching). With resists of this nature, so-called bilevel resists, the generation of highly resolved structures has now become possible using bilayer technology.
A silicon-containing bilevel resist is known for example from U.S. Pat. No. 4,521,274. This resist is a condensation product of formaldehyde and a phenol substituted with silicon, for example trimethylsilylphenol, which means it is a silicon-containing phenol novolak. The resist can also be in the form of a terpolymer of phenol, trimethylsilylphenol, and formaldehyde (see: J. Vac. Sci. Technol., Vol. B3, 1985, pages 306 to 309).
An Si-containing resist for bilayer resist systems, i.e. for two layer technology is also known from J. Electrochem. Soc,, Vol. 132, 1985, pages 909 to 913. This system is a partially trimethylsilylmethylated resorcin formaldehyde resin, i.e. a silicon-containing resorcin novolak.
Silicon-containing positive resists of the above mentioned kind also have the additional advantage that they are soluble in alkaline solutions. This is important for the reason that developing can be carried out in aqueous alkaline solutions, i.e. use of organic developers can be dispensed with. In contrast, positive resists in the form of Si-containing polymethylmethacrylates, as they are known from U.S. Pat. No. 4,481,049 and also resists having substituted silyl residues known from EP Pat. No. 0 096 596 do not have this property.
For bilayer resist systems, further, a positive resist based on silylated polystyrene is known, which has alkylsilyloxy groups (see: A. Heuberger, H. Beneking "Microcircuit Engineering 84", Academic Press Inc., 1985, pages 471 to 481). In addition to its resistance against reactive ion etching in an oxygen plasma, this resist has the property of alkali solubility. Its resolution capability, however, does not meet the demands made of it and the contrast (for UV light) is relatively low, specifically, it is less than 2.
The known Si-containing positive resists, which are sensitive to high-energy radiation, however, do not have all of the required properties. In addition to resistance against reactive ion etching in an oxygen plasma, high resolution capability, and alkali solubility, above all high softening temperature is required. A high softening temperature is important for retaining the profile or the exact size-retaining ability of the resist structures, and in particular during the drying process, as well as during etching in an oxygen plasma, because in the process, the substrate, for example a wafer, is heated. With low softening temperatures, as is the case with the known resists, rounding off of the resist edges occurs at relatively low temperatures.
It is an object of the invention to develop a method of the foregoing kind in such manner that the profile retention or ability to retain exact size of the resist structures is ensured. Other objects include development of a resist material that has resistance against reactive ion etching in an oxygen plasma, high resolution capability, alkali solubility and a high softening temperature.